Well, it's Mark Polesky we both owe.
Without him, all small-time Schemers like me would still be
believing the very plausible explanation in (the 2.12) NR 5.3.4
of why \tweak cannot be used to modify accidentals.

I've not been following this thread very closely,
but it seems like you're suggesting NR 5.3.4 is

not correct. If so, I missed what Mark said.
Could you please say how this section should be
modified or, even better, post a patch to be applied
to changing-defaults.itely.